A tandem pump can discharge the same or different pressurized fluids from the discharge ports of both fluid pumps, or can supply the flow of two pumps using a structure that merges the fluids discharged from both fluid pumps, and this flow is supplied to a single fluid supply subject. A known example of this type of tandem pump is an oil pump which supplies oil for lubricating and cooling the oil gallery in an engine case. With this oil pump, even if the engine which is the drive source has low output and the pump rotational speed is low, the oil discharged from both pumps can be merged together in order to provide sufficient oil flow for lubrication. Furthermore, when the pump rotational speed is high, excessive oil supply can be prevented by running one of the fluid pumps in a no-load condition and supplying the oil from the second fluid pump to the oil gallery, in order to prevent wasting engine power.
In order to efficiently operate the pump based on the pump rotational speed, a tandem pump generally has a no-load valve to put one fluid pump in a no-load condition. When the structure is such that the oil discharged from both fluid pumps is guided such that the fluid supply channels merge, a check valve is provided to prevent oil discharged from the second pump from flowing back from the merge point when the first fluid pump is in a no-load condition. Note, during high speed operation of the pump, only the oil discharged from the second pump is supplied to the oil gallery, but a relief valve is provided to protect the oil pressure system and is set to keep the pressure of oil discharged from that fluid pump below a prescribed pressure.
When a plurality of separate valves are provided, the overall size of the pump equipment will increase, and therefore a valve structure which has the functions of three valves has been proposed, with a spool and tappet provided in line along the discharge channel of one fluid pump (for instance, refer to International Patent Disclosure 06/033207). With this valve structure, the function of a plurality of valves can be achieved using fewer components and the valves for a tandem pump can be made more compact.
With the valve structure shown in International Patent Disclosure 06/033207, when the discharge of oil pressure is below a no-load operation pressure, the oil discharged from one of the oil pumps will merge with the oil discharged from a second oil pump in the internal channel of the spool and will be supplied to the oil gallery. When the discharge of oil pressure reaches the no-load operation pressure, a part which is connected to the discharge opening of one oil pump will be connected to the drain (or to the intake opening of that same pump). Furthermore, the pressure differential between the oil pressure of the oil in the internal channel discharged from the oil pump connected to the drain and the oil pressure of the oil discharged from the second oil pump which flows to the space on the right side of the spool will move the tappet to the left. Therefore, the tappet and the spool will contact, the inside channel will be closed, and all of the oil output from the first oil pump will be discharged to the drain side.
With this valve mechanism, the rate of flow discharged from the pump will be cut to approximately one half immediately after the no-load operation pressure is exceeded, the oil pressure will drop, and may fall below the no-load operating pressure. Therefore the spool and tappet will separate, the oil discharged from both oil pumps will be merged together, and the discharged oil flow will approximately double, and therefore the oil pressure will rise and again reach the no-load operating pressure, and the spool and tappet will make contact. In this manner, when the discharge oil pressure of the pump is close to the no-load operating pressure, the tappet will move back and forth in the axial direction and will repeatedly contact and separate from the spool (chattering), and there is a possibility of causing noise. Furthermore, movement of the tappet back-and-forth is also a cause of pulsing pressure in the supplied oil.